I found this just now.



Mast Vibration

Sleepless Nights!

Anyone who has a vibrating mast knows how much of a nuisance this can be when trying to sleep or otherwise spend time down below. Mast vibration causes a great deal of noise below decks since the hull acts like a sounding board and amplifies the vibration noise.

Vibration is more likely to occur on poorly stayed deck stepped masts, usually with single lower stays which do not provide fore/aft support. Technical Explanation

The technical explanation of mast vibration is something called Karman vortex shedding. Basically, air flowing over the mast does not re-attach as it would over an airfoil and this causes regions of low pressure on the leeward side. These regions are unstable and tend to move up and down as the vortex shedding occurs. The rate at which the low pressure regions move is a function of windspeed, mast diameter and shape. When the natural frequency of the mast section coincides with the frequency of the vortex shedding, a resonance can occur which results in an marked increase in amplitude and severe vibration can occur. This phenomenon often occurs at quite low windspeeds (4-12) knots.

Vortex shedding is common on tall slender chimneys - If you look at any industrial plant, you will likely see a chimney with spirals welded on. These spirals are intended to move the low pressure regions from the leeward side to the windward side and thus overcome the vertical movement of the eddies. Quite a lot is known about Karman vortices, but this has not always been useful in yacht design. Solutions?

At the design stage, the designer should consider the following:
Mast Support - Keel vs Deck or at least ensure the mast step is well supported.
Mast staying - Use fore/aft lowers or swept back spreaders to better provide fore/aft staying
Mast Section - Analyse the mast natural frequency vs Karman vortex shedding.
Mast Shape - It could be that some mast shapes are better than others.
Once you have this problem, there are a few things that could be tried:
Mast Step 1 - Ensure that the mast base is fully supported on the mast step and not bearing on a point.
Mast Step 2 - Ensure that there is adequate support under the mast step - there is sometimes a gap.
Rig Tension 1 - If you have fore/aft lowers or a baby stay, try varying the tension.
Rig Tension 2 - Try adjusting upper shroud tension - This affects the natural frequency of the mast.
Temporary support - Tie a line from the Jib tack to the spinnaker eye or simply around the mast.
New Stays - add fore/aft support by adding baby stay or additional lowers.
Add a Vortex Fence - A narrow stiff (2 to 4") strip raised to say 2/3 of the mast height in the mast slot.
Try using your mainsheet to tension the topping lift and provide some forward boom force to stiffen the lower mast.
Try inducing some mast bend with the backstay adjuster - this will stiffen the mast.
Summary

This an annoying and difficult problem to solve - There is no simple answer. The only way is to keep trying ideas until something works for you. Good Luck!

Bud,